Chemical Analysis of Actinolite from Reflectance Spectra Jonx F. Musunn
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American Mineralogist, Volume 77, pages 345-358, 1992 Chemical analysis of actinolite from reflectance spectra Jonx F. Musunn Department of Geological Sciences,Box 1846, Brown University, Providence,Rhode Island 02912, U.S.A. Ansrn-c.cr Reflectancespectra of medium (0.005 pm between 0.3 and 2.7 pm) and high (0.0002 pm between1.38 and 1.42pm and 0.002 pm between2.2 and2.5 pm) spectralresolution of 19 minerals from the tremolite-actinolite solid solution seriesare used to characteiue systematic variations in absorption band parametersas a function of composition. The medium-resolution data are used to characterizebroad absorptions associatedwith elec- tronic processesin Fe3+and Fe2*, and the high-resolution data are used to characterize overtones ofOH vibrational bands. Quantitative analysesofthese data were approached using a model that treats absorptions as modified Gaussiandistributions (Sunshineet al., 1990). Each spectrum was deconvolved into a set of additive model absorption bands superimposedon a reflectancecontinuum. For absorptionsassociated with electronic pro- cesses,the model bands were shown to provide estimates of the Fe and Mg mineral chemistry with correlations of greater than 0.98. The systematic changesin the energy, width, intensity, and number of OH overtone bands are easily quantified using the mod- ified Gaussian model, and the results are highly correlated with the FelMg ratios of the samples.These results indicate that the modified Gaussianmodel can be used to quantify objectively the bulk chemistry of the minerals of the tremolite-actinolite solid solution seriesusing high-resolution reflectancespectra ofthe OH overtone bands. INrnooucrroN into realistic probability distributions. In the analyses Extraction of quantitative geochemicalinformation is presentedhere, a similar approach is applied to the ab- vibrational pro- an important goal in reflectancespectroscopy, both as a sorptions associatedwith electronic and the tool for rapid and nondestructivelaboratory analysesand cessesin calcic amphiboles and, more specifically, for interpretation of remotely obtained observations of minerals of the tremolite-actinolite-ferroactinolite solid very in the Earth and other solid bodies in the solar system.The solution series. Calcic amphiboles are common chemical and crystallographicbases for the interpretation terrestrial geologic terranes, especially mafic and ultra- ofabsorption bands in reflectancespectra are reasonably mafic terrane,and specificcompositions often can be used well understood(e.g., Burns, 1970;Farmer,1974;Hunt, to infer physical conditions of mineral and rock forma- (e.g., Annersten, 1977). The assignment of a particular absorption to a tion Laird and Albee, l98l; Skogbyand goal specificcrystallographic site geometryand absorbingspe- 1985).The of this investigationis not only to model cies have proven consistent acrossa large range ofcom- the specificabsorptions associated with calcic amphiboles provide processes positions and crystal structures.This knowledgehas been but also to insight into the absorption general min- extremely valuable for the remote identification of min- in that can be used in the analysisof other groups. eral specieson other planets(e.g., Pieters, 1986; McCord erals and mineral Background discussionsofac- pro- et al., 1982).However, there have been few detailed anal- tinolite crystal structure, chemistry, and absorption presented yses to determine quantitative associationsbetween the cesses are below, followed by a detailed position, shape,and strength of absorption bands in re- discussion of the modeling approach. The results of the presented flectancespectra and the chemical composition of min- modeling of the reflectance spectra are then model erals.A sound understandingofthe relationship between with an emphasistoward the associationbetween absorption band parametersand specific mineral chem- parametersand sample chemistry. istry would have a broad range of applications in the reduction and analysis of remotely obtained and labora- Actinolite crystal structure and chemistry tory reflectancespectra. Understanding of the variability of the reflectance One desiresa method of deconvolving reflectancespec- spectra shown in Figure I requires a basic knowledge of tra into concentrationsofabsorbing speciesthat produces crystal structure and chemistry. The idealized actinolite quantitative results and is generalin approach. Sunshine chemical formula, Car(Mg,Fe)rSisorr(OH)r,is rarely en- et al. (1990) have made some progressin this area by countered with natural samples.There are generally ap- deconvolving Fe2*absorptions in pyroxenesand olivines preciable amounts of Al3*, Fe3*,Mn, Cr, and Na in these 0003404x/92l0304-o345$02.00 345 346 MUSTARD: ACTINOLITE ANALYSES FROM REFLECTANCE SPECTRA 1 o o 17- G' 4' C) (D 7 (r(l) 1 5 2 0 1'/ FawavetSfiogtn, 112 'ow.rrt3tf,gtn .pf, Infm in a generalprogression from high-Fe samplesnear the bottom to low-Fe samplesnear the top. (a) Data obtained at a sampling resolution of 0.005 pm. The number adjacent to each spectrum 0.30 0.60 o.g0 1.20 t.50 1.80 z1o 240 near 0.6 pm refersto the samplenumber, and the numbers near WavelengthIn /rm 1.5 pm give the percent reflectanceof each sample at 1.5 pm. (b) Reflectancespectra obained at a sampling resolution of 2 A Fig. 1. Reflectancespectra of 19 samplesfrom the tremolite- between 1.38 and 1.43 pm; (c) data obtained at a sampling res- actinolite solid solution series.All data have beenoffset for clar- olution of 0.002 pm between 2.2 and 2.45 rrm. The numbers ity approximately 50/orelative to each other. The numbers on betweenb and c indicate the samplenumber of the spectrajoined the left side of each plot indicate the reflectancescale. There is by the solid lines. minerals. The actinolite crystal structure is monoclinic shown that Fe2* is enriched in Ml and M3 relative to with space group C2/m and consists of two unique tet- M2 in calcicamphiboles (Burns and Prentice,1968; Ernst rahedralsites (Tl, T2), four unique octahedrallycoordi- and Wai, 1970;Wilkens et a1.,1970; Wilkens, 1970).Mg nated sites(Ml, M2, M3, and M4), and one l2-coordi- showsa site preferencethat is complementaryto Fe2*.Ca nated site (A) (Ghose,l96l). almost exclusively occupiesthe M4 site, but there is rare- Site populations are complex in amphiboles (seeHaw- ly enough Ca to fill this site completely. Vacant M4 sites thorne, 1983, for a review) and are important for the are preferentially filled by available Mn and Fe2*. interpretation and quantifi cation of actinolite reflectance spectra.Al3* shows a site preferencefor Tl when in tet- Sourcesof absorption in calcic amphiboles rahedral coordination and the M2 site when in octahedral Three basic processesare responsible for the suite of coordination. Fe3*,like Alr*, exhibits a preferencefor the absorptions observed in the spectra shown in Figure l. M2 site (Wilkens, 1970; Hawthorne, 1983; Skogbyand Excitation of d-orbital electronsin transition metal ions, Annersten,1985). However, local chargebalance can in- particularly Fe, is primarily responsiblefor the broad ab- fluencethese general distributions. Although Fe2+can oc- sorptionscentered between 0.85 and 1.2 p.m.The broad cupy any ofthe four octahedral sites and the specificor- absorption centered near 2.45 pm is largely due to an dering among thesesites is complex, severalstudies have electronic transition. Absorptions involving transfer of MUSTARD: ACTINOLITE ANALYSES FROM REFLECTANCE SPECTRA 347 electrons between anions and cations generally exhibit Burns and Strens,1966; Bancroft and Burns, 1969;Wil- band minima at wavelengthsless than about 0.5 pm, but kens, 1970;Burns and Greaves,l97l;Law,1976). The wings ofthese absorptions do extend through the visible wavelength position of the OH- stretching fundamental to the near infrared. Cation-to-cation chargetransfer ab- dependson the cation occupancyof the two M I and one sorptions can have well-defined visible to near infrared M3 sites as a unit. For an ideal actinolite solid solution, absorptions. Finally, overtones and combination over- there are four unique distributions: A : (MgMgMg), B : tones of fundamental vibrational bands associatedwith (MgMgFe), C : (FeFeMg),and D : (FeFeFe).Since each OH ions account for the narrow featuresnear 1.4 rrm ond results in a distinct absorption, there are four possible at wavelengthslonger than 2.0 pm. Details of each cate- OH- stretching fundamentals in a simple Fe-Mg amphi- gory of absorption and relationship to cation and anion bole system.Additional bands can be expectedif the Ml site occupancyare discussedbelow. and M3 sitesare occupiedby other ions (e.g.,A13*, Fe3*) Electronic transitions. The energy ofelectronic transi- or the A site is occupied by Na or K. tion absorptions is determined by the crystal field split- The narrow absorptions between 2.2 and 2.5 pm (see ting, whereasthe intensity or strength ofthe absorptions Figs. la and lc) are combination overtonesinvolving OH- is governedby probability considerationsdetermined pri- stretching vibrations and metal-OH- bending modes marily by site distortion and symmetry (Burns, 1970; (Hunt, 1977).Although the location of the fundamental White and Keester,1967). The broad, indistinct absorp- metal-OH bendingmodes is reasonablywell known (500- tions between0.85 and 1.2 p.mare generallyassigned to 800 cm-'or 12-15 pm), there has been little work to Fe2* electronictransitions in Ml, M2, and M3 (Haw- quantify relationships between parameters that charac- thorne, 198l). However, little